Process for the deposition of platinum-rhodium layers having improved whiteness

ABSTRACT

The present invention relates to an electrochemical process for the deposition of coatings comprising an alloy of platinum and rhodium on, in particular, decorative articles. The process of the invention is characterized in that defined conditions are employed under which the electrolytically deposited layer has, contrary to expectations, a high whiteness which comes extraordinarily close to the appearance of silver.

The present invention relates to an electrolytic process for the deposition of coatings comprising an alloy of platinum and rhodium on, in particular, decorative articles. The process of the invention is characterized in that the electrolytically deposited layer has, contrary to expectations, a high whiteness which comes extraordinarily close to the appearance of silver.

It is known that silver articles in particular tarnish over time and thus become unsightly. The use of silver articles, especially in the decorative sector, is therefore subject to restrictions. Among other things, frequent cleaning of the article is important to maintain its appearance and aesthetics. For this reason, it would be advantageous for the silver article to be able to be coated with a protective coating which has a similar appearance and is inert under the prevailing conditions. This would be of particular interest for the coating of eating utensils and jewelry.

Coatings can be applied to such articles in various ways. Coating by electrolytic methods is an option for electrically conductive articles. Different coating processes are usually used in the electroplating industry as a function of the type and nature of the parts to be coated. The processes differ, inter alia, in respect of the current densities which can be employed. Mention may be made of essentially 3 different coating processes.

-   -   1. Drum coating for loose material and mass-produced parts:         -   in this coating process, relatively low working current             densities are employed (order of magnitude: 0.05-0.5 A/dm²)     -   2. Rack coating for individual parts:         -   in this coating process, medium working current densities             are employed (order of magnitude: 0.2-5 A/dm²)     -   3. High-speed coating for strips and wires in continuous plants:         -   in this coating field, very high working current densities             are employed. (Order of magnitude: 5-100 A/dm²)

The coating of metals with an alloy of rhodium and platinum is already known in the prior art. DE-B 1229816 describes a coating comprising a metal alloy of the platinum group metals and containing at least 50% by weight of platinum, at least 50% by weight of rhodium or at least 50% by weight of platinum together with rhodium for an anode. This document merely mentions that the platinum alloy can be obtained, inter alia, by electroplating. The coating formed in this way is intended to protect the anode against corrosion.

Processes for the deposition of a platinum-rhodium alloy from basic baths are likewise known. In U.S. Pat. No. 4,427,502, polyamine ligands are used for complexing the noble metals in the solution. Alloys which can contain at least 10 mol % of platinum together with other noble metals such as rhodium are obtained. GB 348919 discloses the electrolytic deposition of a platinum-rhodium alloy comprising 90% by weight of platinum and 10% by weight of rhodium from an ammoniacal electrolyte containing nitrite ions, palladium chloride and rhodium chloride.

In addition, such alloys can also be obtained from acidic electrolytes. Thus, U.S. Pat. No. 3,671,408 describes how an alloy of platinum and rhodium which is either rich in platinum or rich in rhodium can be deposited onto transition metals from a bath containing sulphuric acid and sulphamic acid. An acidic bath containing a platinum complex consisting of Pt(NH₃)₂(NO₂)₂ and rhodium sulphate is used in this process. It is stated that the rhodium-rich bath is particularly suitable for the coating of silver articles since very white deposits are achieved here. The alloy deposited in this way contains more than 90% by weight of rhodium. Deposition is carried out, inter alia, from a bath containing an about 10-fold excess of rhodium.

U.S. Pat. No. 3,748,712 describes the coating of silver articles with an alloy which has a thickness of from 1 to 50 microinches and contains from 91 to 99% by weight of rhodium and from 1 to 9% by weight of platinum. The electrolytic bath is said to contain from 0.5 to 20 g/l of the sulphate compound of the noble metals and be operated at a pH of from 1 to 3.5. The current density to be applied is reported as being from about 0.5 to 4.3 A/dm².

It is common to both publications that they propose a very rhodium-rich alloy as coating in order to be able to achieve the required whitenesses. However, the price of rhodium is exorbitantly high compared to platinum. Unfortunately, rhodium is not only the whitest platinum group metal but also the most expensive. Thus, the cost of one gram of rhodium is about 5 times that of one gram of platinum. For this reason, many jewelry manufacturers are attempting to get away from the expensive rhodium and replace it by the cheaper platinum. However, this works only when the differences in colour and brightness from normal silver are not too obvious.

It would therefore be desirable to develop a process which allows coatings having a whiteness comparable to that of silver to be produced using less rhodium. This should be superior to the processes known from the prior art. In particular, the coatings obtained according to the invention should be more advantageous to produce with not significantly impaired or comparably good or even improved adhesion, tarnishing resistance and also colour and colour stability.

These objects and further objects which are not mentioned in the prior art but can be discerned by a person skilled in the art in an obvious fashion are achieved by a process having the features of the present Claim 1. Preferred embodiments of the process of the invention are defined in the claims dependent on Claim 1.

The stated object is particularly surprisingly achieved in a simple but nonetheless advantageous manner by the electrolysis in a process for the electrolytic deposition of an alloy of platinum and rhodium on, in particular, decorative articles, where the rhodium content of the alloy ranges from at least 40% by weight to not more than 85% by weight, preferably from 45% by weight to 80% by weight, particularly preferably from 48% by weight to 70% by weight, very particularly preferably from 49% by weight to 60% by weight, and extremely preferably about 50% by weight, being carried out at a pH of ≦0.1 and a current density of ≧0.2 A/dm² in an aqueous solution comprising:

a) 0.4-5.0 g/l of platinum ions, b) 1.0-5.0 g/l of rhodium ions and optionally c) one or more additives selected from the group consisting of supporting electrolytes, brighteners, surfactants, wetting agents and ligands which complex noble metals and stabilisers, as electrolytic bath. The process of the invention makes it possible to make do with less rhodium in a platinum-rhodium coating and nevertheless obtain firmly adhering, stable coatings which are nevertheless attractive and look like silver on appropriate goods and articles. The coatings are generally 2 lightness units (according to the Cielab system—http://www.cielab.de/) lighter than would have been expected theoretically from the proportion of pure rhodium.

Furthermore, the throwing power of the platinum-rhodium electrolyte used in the process of the invention is particularly good. In electroplating, the throwing power is the ability of an electrolyte to achieve an improved coating distribution on the workpiece to be coated despite a non-uniform current distribution. In detail, a distinction is made between macro and micro throwing power. Macro throwing power is the ability of an electrolytic bath to achieve an approximately uniform layer thickness over the entire surface of the workpiece including the lower regions. Micro throwing power is the ability of an electrolytic bath to deposit metal in pores and scratches.

In the electrolyte of the invention, the metals rhodium and platinum to be deposited are present in dissolved form as their ions. They are preferably introduced in the form of water-soluble salts which are preferably selected from the group consisting of pyrophosphates, carbonates, hydroxidecarbonates, hydrogencarbonates, sulphites, sulphates, phosphates, nitrites, nitrates, halides, hydroxides, oxidehydroxides, oxides and combinations thereof. Very particular preference is given to the embodiment in which the metals are used in the form of salts with ions selected from the group consisting of pyrophosphate, carbonate, sulphate, hydroxidecarbonate, oxidehydroxide, hydroxide and hydrogencarbonate. The type and amount of salts introduced into the electrolyte can likewise be decisive for the colour of the resulting decorative coating and can be set according to customer requirements. The metals to be deposited are, as indicated, present in ionically dissolved form in the electrolyte for the application of decorative layers on jewelry, consumer goods and industrial articles. The ion concentration of platinum can be set in the range 0.4-5.0 g/l of electrolyte, preferably from 0.5 to 4.0 g/l of electrolyte, and the ion concentration of rhodium can be set in the range 1.0-5.0 g/l of electrolyte, preferably 1.5-2.5 g/l of electrolyte. In the upgrading of goods, particular preference is given to introducing the metals to be deposited as sulphate or phosphate, carbonate or hydroxidecarbonate so that the resulting ion concentration is in the range from 0.5 to 1.0 gram of platinum and from 1.0 to 2.0 gram of rhodium, in each case per litre of electrolyte.

The electrolyte can contain one or more of the additives indicated. Apart from the metal salts to be deposited, further additives, including organic additives, which perform functions as supporting electrolytes (e.g. H/Na/K/NH₄ sulphates, phosphates, sulphonates or mixtures thereof (Handbuch der Galvanotechnik, Carl Hanser Verlag, 1966)), brighteners (including aromatic or heterocyclic sulphonic acids, selenous acid, aluminium, magnesium (Galvanische Abscheidung der Platinmetalle, reprint by the DGO from Issue No. 2+4, Volume 91, 2000)), wetting agents (such as polyfluorinated sulphonic acids, aliphatic sulphates (A. v. Krustenstjern, Edelmetallgalvanotechnik 1970, Eugen G. Leuze Verlag, Saulgau)) or stabilisers (e.g. sulphonic acids, sulphites (A. v. Krustenstjern, Edelmetallgalvanotechnik 1970, Eugen G. Leuze Verlag, Saulgau)), ligands which complex noble metals (e.g. sulphates, phosphates, methanesulphonates or mixtures thereof (Edelmetalltaschenbuch Degussa, 2^(nd) edition, 1995, Hüthig Verlag, Heidelberg)) or surfactants (including anionic, cationic, nonionic surfactants (A. v. Krustenstjern, Edelmetallgalvanotechnik 1970, Eugen G. Leuze Verlag, Saulgau)).

The addition of brighteners and wetting agents is particularly preferred only in the case of the appearance of the decorative layers to be deposited having to meet special requirements. These make it possible to adjust, in addition to the colour of the coating which depends mainly on the ratio of the metals to be deposited (FIG. 3), the brightness of the layer in all gradations between matt silk and high gloss. Preference is also given to adding one or more compounds selected from the group consisting of monocarboxylic and dicarboxylic acids, alkanesulphonic acids, betaines, sulphamic acids, sulphites, selenous acid and aromatic nitro compounds. These compounds act as electrolyte bath stabilisers or as brighteners. Particular preference is given to using oxalic acid, alkanesulphonic acids, in particular methanesulphonic acid, or nitrobenzotriazoles or mixtures thereof. Suitable alkanesulphonic acids may be found in EP1001054. A possible carboxylic acid is, for example, citric acid and its Na/K salts (Galvanische Abscheidung der Platinmetalle, reprint by the DGO from Issue No. 2+4, Volume 91, 2000). Betaines to be used are preferably those which may be found in WO2004/005528. Particular preference is given to using those described in EP636713. In this context, very particular preference is given to using 1-3(3-sulphopropyl)pyridinium betaine or 1-(3-sulphopropyl)-2-vinylpyridinium betaine.

Furthermore, a supporting electrolyte can be added to the electrolyte in the process of the invention. Possible supporting electrolytes are alkali metal or alkaline earth metal salts with anions such as pyrophosphates, carbonates, hydroxidecarbonates, hydrogencarbonates, sulphites, sulphates, phosphates, nitrites, nitrates, halides, hydroxides or carboxylate anions, phosphonate anions, sulphonate anions. Particular mention may be made in this context of: sulphuric acid and phosphoric acid which are added in excess to the reaction in the production of rhodium sulphate or phosphate from, for example, rhodium oxide hydrate (Galvanische Abscheidung der Platinmetalle, reprint by the DGO from Issue No. 2+4, Volume 91, 2000).

Surfactants (e.g. anionic, cationic and/or nonionic surfactants, with and without polyfluorinated substituents, which withstand the very low pH in the long term (electroplating chemicals, TIB Chemicals AG, Mannheim)) can also be added.

The application of the coating to decorative articles, consumer goods and industrial articles using the electrolyte according to the invention is carried out, as indicated, in an electrochemical process. It is important here that the metals to be deposited are permanently kept in solution during the process, regardless of whether electroplating is carried out in a continuous or batch process. To ensure this, the electrolyte according to the invention can contain complexing agents. As ligands which complex the noble metals, mention may also be made of those having sulphur atoms or phosphorus atoms e.g. sulphuric acid or phosphoric acid which are added in excess to the reaction in the production of rhodium sulphate or phosphate from, for example, rhodium oxide hydrate (Galvanische Abscheidung der Platinmetalle, reprint by the DGO from Issue No. 2+4, Volume 91, 2000).

The amount of compounds which complex noble metals in the electrolyte can be set in a targeted manner by a person skilled in the art. It is limited by the fact that the concentration in the electrolyte should be above a minimum amount in order to bring about the effect concerned to a sufficient extent.

The pH of the electrolyte is in the range of ≦1 required for this electroplating application. A lower limit is set by the fact that the electrolyte tends to be unstable at pH values which are too low. Preference is therefore given to a range of 0-0.8 and very particularly preferably about 0.2. Acidification of the electrolyte can generally be carried out using inorganic acids. Preference is given to using, inter alia, sulphuric acid for this purpose. In a very particularly preferred embodiment, the aqueous electrolytic bath is acidified using up to 100 ml/l of conc. sulphuric acid.

The process of the invention can be operated at a temperature which a person skilled in the art will choose on the basis of his general technical knowledge. Preference is given to a range from 20 to 70° C. in which the electrolytic bath is maintained during the electrolysis. Greater preference is given to selecting a range of 30-50° C. The process is especially preferably carried out at a temperature of about 45°.

When the process of the invention is employed, it is possible to use various anodes. Preference is given to insoluble anodes. As insoluble anodes, use is advantageously made of anodes composed of a material selected from the group consisting of platinised titanium, graphite, iridium-transition metal mixed oxide and a specific carbon material (“diamond-like carbon”, DLC) or combinations of these anodes. Preference is also given to mixed oxide anodes (MMO) composed of iridium-ruthenium mixed oxide, iridium-ruthenium-titanium mixed oxide or iridium-tantalum mixed oxide. Further materials may be found in Cobley, A. J. et al. (The use of insoluble Anodes in Acid Sulphate Copper Electrodeposition Solutions, Trans IMF, 2001, 79(3), pages 113 and 114). Very particular preference is given to using an MMO of the type Platinode® 177 (which can be procured from Umicore Galvanotechnik GmbH).

An important advantage of the present invention is that the deposition of the alloy composition does not alter significantly over a wide current density range at or above 2 A/dm² (FIG. 2). This also results in a surface quality which appears to be sufficiently homogeneous even at current densities which are relatively high for rack applications. A person skilled in the art will choose the current density range on the basis of economic and technical boundary conditions so as to obtain the result which is considered to be optimal. It is advantageous to choose a current density of not more than 7.0 A/dm², preferably 6.0 A/dm², and the current density is particularly preferably in the range 3.0 A/dm²-4.0 A/dm².

Platinum ions can also be used in previously complexed form in the process of the invention. Commercially available compounds of this type are, for example, ammonia complexes of platinum [Pt(NH₃)₄SO₄] or [Pt(NH₃)₂SO₄]. Since nitrogen-containing ligands can be present in the electrolyte, these are preferably introduced in the form of the corresponding platinum complexes into the electrolyte. The platinum ions are therefore preferably used in the form of complex salts with nitrogen ligands such as ammonia, monoamines or oligoamines. The use of polydentate ligands, in particular ligands based on diamines, triamines or tetramines, is advantageous here. Particular preference is given to ligands having from 2 to 11 carbon atoms. Very particular preference is given to using ligands selected from the group consisting of ethylenediamine, trimethylenediamine, tetramethylenediamine, pentamethylenediamine, hexamethylenediamine, 1,2-propylenediamine, trimethylenetetramine, hexamethylenetetramine. The greatest preference is given to ethylenediamine (EDA) for this purpose.

The process of the invention allows the deposition of white platinum-rhodium layers, in particular on decorative metal articles made of silver, in an economically advantageous way. The whiteness (lightness) of an about 50:50 alloy is significantly increased over that which would have been expected theoretically and approximates that of alloys containing >90% of rhodium metal (Overview 1).

Overview 1:

Abrasion behaviour and savings potential of the alloy system Rh/Pt:

Density Price [ 

/g]*) [g/cm³] Platinum  44.25 

21.45 Rhodium 209.60 

12.41 *)Price on the day (Mar, 07, 2008)

Rhodium-platinum alloy:

Rhodium Colour Colour Colour Saving content [%] (L* value) (a* value) (b* value) [%] 0 86.5 0.4 5.4 64 20 86.6 0.4 5.3 44 30 87.9 0.4 4.8 36 40 88.5 0.4 4.5 30 50 89.0 0.5 4.3 24 60 88.9 0.5 4.2 19 70 88.8 0.6 4.2 15 80 88.7 0.7 4.2 10 90 88.9 0.8 5.8 6 100 89.5 0.8 3.8 0 (Note: Variations or fluctuations in the measured values are due to small weighing and measurement inaccuracies)

It can be seen that the colour and the optical appearance, e.g. the shininess of the layers, is at least not significantly poorer than that of a pure rhodium layer. This enables a dramatic proportion of expensive rhodium to be saved. This would not have been expected from the prior art.

EXAMPLE

Electrolytically deposited platinum layers having improved whiteness and electrolyte for this purpose

Electrolyte Composition: Platinum: 0.6 g/l Rhodium: 1.5 g/l

Sulphuric acid, conc.: 40 ml/l (=about 70 g/l) Total acid: 80 g/l

Operating Conditions: Temperature: 45° C. pH: 0.25 (at 45° C.) or 0.2 (at 25° C.)

Density: 1.046 g/cm³ (at 45° C.) or 1.051 g/cm³ (at 25° C.) Current density: 2.0 A/dm² (0.25-5.0 A/dm²) Anodes: MMO (type Platinode® 177; titanium anodes coated with mixed metal oxide for strongly acidic rhodium or platinum electrolytes, commercially available from Umicore Galvanotechnik GmbH) Deposition rate: about 0.084 μm/min (at 2.0 A/dm²) Deposition yield: about 7.1 mg/Amin (at 2.0 A/dm²) Alloy (about): Pt:Rh=50:50 (at 2.0 A/dm²)

White, shiny layers could be produced using an electrolyte operated at these parameters. Their colour (according to CIEL*a*b*) was determined using a colour measuring instrument from Xrite (model SP 62). Here, the L* value determines the lightness of the layer (corresponds to the percentage reflection of light incident on the layer. L*=0 means totally black, L*=100 means complete reflection of the light (http://www.cielab.de/).

A sharp increase in the lightness (whiteness) is conspicuous when the current density is increased from 0.25 to about 2.0 A/dm². At current densities >2.0 A/dm², the lightness increases only slightly or remains constant (FIG. 2).

DESCRIPTION OF THE FIGURES

FIG. 1 The lightness (whiteness) of the coating is shown here as a function of the proportion of rhodium in the alloy. It can be seen that above a proportion of 40% of rhodium, the lightness of the alloy increases above the value to be expected theoretically.

FIG. 2 The lightness (whiteness) of the coating is approximately constant when the applied current density is varied, as long as a current density of ≧2 A/dm² is selected.

FIG. 3 The lightness (whiteness) of the alloy no longer increases above a proportion of about 50% of rhodium. Small differences in the alloy composition in this range are therefore of little significance.

FIG. 4 The alloy composition remains approximately constant over a wide current density range from ≧2 A/dm² upwards. 

1. Process for the electrolytic deposition of an alloy of platinum and rhodium on, in particular, decorative articles, where the rhodium content of the alloy ranges from at least 40% by weight to not more than 85% by weight, wherein, the electrolysis is carried out at a pH of ≦1 and a current density of ≧2 A/dm² in an aqueous solution comprising: a) 0.4-5.0 g/l of platinum ions, b) 1.0-5.0 g/l of rhodium ions and optionally c) one or more additives selected from the group consisting of supporting electrolytes, brighteners, surfactants, wetting agents, ligands which complex noble metals and stabilisers as electrolytic bath.
 2. Process according to claim 1, wherein, the aqueous electrolytic bath is acidified with up to 100 ml/l of conc. sulphuric acid.
 3. Process according to claim 1, wherein, the temperature is in the range from 20 to 70° C. during the deposition.
 4. Process according to claim 1, wherein, a mixed metal oxide anode is used as anode.
 5. Process according to claim 1, wherein, the current density is not more than 7.0 A/dm².
 6. Process according to claim 1, wherein, the platinum ions are used in the form of complex salts with nitrogen ligands such as ammonia, monoamines or oligoamines.
 7. Process according to claim 2, wherein, the temperature is in the range from 20 to 70° C. during the deposition.
 8. Process according to claim 2, wherein, a mixed metal oxide anode is used as anode.
 9. Process according to claim 3, wherein, a mixed metal oxide anode is used as anode.
 10. Process according to claim 2, wherein, the current density is not more than 7.0 A/dm².
 11. Process according to claim 3, wherein, the current density is not more than 7.0 A/dm².
 12. Process according to claim 4, wherein, the current density is not more than 7.0 A/dm².
 13. Process according to claim 2, wherein, the platinum ions are used in the form of complex salts with nitrogen ligands such as ammonia, monoamines or oligoamines.
 14. Process according to claim 3, wherein, the platinum ions are used in the form of complex salts with nitrogen ligands such as ammonia, monoamines or oligoamines.
 15. Process according to claim 4, wherein, the platinum ions are used in the form of complex salts with nitrogen ligands such as ammonia, monoamines or oligoamines.
 16. Process according to claim 5, wherein, the platinum ions are used in the form of complex salts with nitrogen ligands such as ammonia, monoamines or oligoamines. 